The invention can relate to any type of electromagnetic radiation but has particular utility for X-rays. Electromagnetic waves having wavelengths in the range of 0.01 to 10 nm are known as X-rays. X-rays have energies in the range of 120 eV to 120 keV.
It is known to image objects using X-rays by measuring X-ray absorption. Typically this involves having an X-ray source and detector with a sample in between them. The primary X-ray beam is directed towards and hits the sample, with some of the X-ray radiation being absorbed, a smaller amount being scattered and the remainder going on to hit the detector.
X-ray absorption imaging is reasonably effective for imaging the shape of a structure, however reliance on the absorption characteristics of the objects under inspection produces low overall accuracy in terms of material identification. For example dual-energy X-ray imaging exploits the difference in atomic cross section between the photoelectric absorption and the Compton scattering processes inferred by the relative change in magnitude of a high-energy X-ray signal and a low-energy X-ray signal. Consequently an appropriately calibrated X-ray system may be employed to broadly discriminate an inspected object into a limited number of material classes. The discrimination information may be presented to the human observer by colour coding the resultant X-ray images. Thus, for example, security personnel in an airport might review the contents of bags going through an X-ray scanner and can look at the pseudo colours displayed as well as the shape to identify anything suspicious.
Such X-ray absorption techniques can be used in real time and on every day objects, however such techniques allow only for crude discrimination of materials. Existing absorption techniques are not adequate for distinguishing between materials that have similar chemical signatures, or for detection of objects that have flat shapes. For example, X-ray absorption imaging is unsuitable for the identification of precise material useful to find explosive substances or contraband drugs.
It is also known to solve the structure of a crystal by analysing the scattering of X-rays through a crystal, for example by analysing the diffraction pattern produced. This is known as X-ray crystallography.
A small portion of a primary X-ray beam incident onto a crystal is scattered at measurable angles if its wavelength is similar to the lattice distances (or d-spacing) present in the crystalline material under inspection. For ideal, polycrystalline materials interrogated by pencil beams, the photon scatter follows a cone distribution, with the source of the scattering at the cone apex. These “Debye cones” form circular patterns when they intersect a flat detector normally. These circles have a common centre coincident with that of the incident beam position on the detector. The angular distribution of the scattered intensity is unique to each different crystal structure and thus can be used to identify a material and determine characteristics such as lattice dimensions, crystallite size and percentage crystallinity. The key relationship between the lattice spacing (d), and the scatter angle (θ) is embodied within the well known Bragg condition: λ=2d sin θ (where λ is the X-ray wavelength).
X-ray crystallography allows for the structure of a large number of molecules of different materials including inorganic compounds to be determined. Ordinarily this is done with single crystals though it is possible to obtain significant information from powdered material or from thin films. This technique allows a large amount of information about materials to be determined. However, even where powders rather than single crystals are used it is a requirement to prepare a custom made small sample which is then bombarded with X-rays perhaps over many hours to provide adequate detection and subsequent analysis of the diffraction pattern.
Conventional powder diffractometers utilise detectors to scan and measure a portion of the resultant diffraction pattern. This angular dispersive technique usually employs monochromatic X-rays. Data collection and analysis have been based mainly on one-dimensional (1D) intensity profiles obtained with scanning point detectors or linear detectors. The linear detector is often referred to in the field as a position sensitive detector or PSD. The use of 2D image sensors (array or area detectors) may be used to speed up the collection of data in comparison to point or line detectors. However the collection process is still relatively slow.
Some of the commonly used X-ray scattering techniques are: single crystal diffraction (SCD), X-ray powder diffraction (XRPD), high-resolution X-ray diffraction (HRXRD), X-ray reflectometry (XRR) and small angle X-ray scattering (SAXS). In general diffractometers are laboratory instruments which are designed for off-line inspection requiring relatively long periods of data collection from carefully prepared samples, because the amount of radiation that is scattered is relatively low and therefore long integration periods are required in order to accumulate a sufficient amount of signal for accurate measurement. For this and other reasons X-ray crystallography can be a very effective technique in laboratories for slow analysis but would not generally be suitable for every day objects or for use in “real time” or “on-line” inspection applications.
Bragg diffraction may occur whenever the wavelength of incident radiation is of a similar magnitude to the lattice spacing of a crystal under analysis, and so crystallography techniques are not limited to X-rays. Particles such as neutrons or electrons can be used if at the correct energy; as well as other electromagnetic radiation.
An alternative technique is disclosed in WO 2008/149078, which is incorporated herein by reference, as well as being illustrated in FIGS. 2-4. The output of an X-ray source is configured, for example by an annular collimator, to form a curtain of X-ray radiation, which can be tubular and/or cone shaped for example. A detector is placed at a position where the Debye cones of X-ray radiation emitted from a sample overlap to form regions of increased intensity, which leads to increased sensitivity and better material discrimination. The technique can be performed over a much shorter time period than that of standard X-ray crystallography.
However, improvements in imaging and/or material detection are still highly sought after.